Push-pull amplifier



Patented Apr. 23, 1935 I y PUSH-PULL AmLIFIER Raymond 0. Wise, Orange,N.y J. assignor tov Bell l Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application June 13,1934, SerialNo. 730,387

. 6 Claims.Vv (Cl. 179-1171).

This invention relatesV to push-pull vacuum tube circuits and moreparticularly to means' 'for automatically balancing said circuits.Themain object cf the invention is to maintain an automatic balancebetween the direct currents in the anode circuits of a pair Yof vacuumtubes.

In accordance with the invention, theequalization of the space currentsof a pair of vacuum tubes connected in push-pull is carried out underthe control ofa differential voltage that is set up whenever the spacecurrents are unbalanced. The diiferential voltageis introduced in theinput circuit of the tubes in serial relation to the two grids, causingone grid to be made more positive .and the other more negative withrespect to the cathodes. It is so arranged that the application of thesegrid voltages causes the larger space current to decrease and theAsmaller to increase until an equilibrium is set up in which the spacecurrents are substantially equal again, although both may have departedfrom their initial value. 1

The invention is applicable to amplifiers Ywhose filament current supplyis either direct current or alternating current provided the followingconditions are observed: (-1) If direct current is used, separatebatteries should be employed .for the two laments; (2) if directlyheated filament type cathode tubes are to be operated'on alternatingcurrent through a power transformer, separate filament current windingsshould be provided for the two tubes; (3) if indirectly heated cathodextubes are employed both heaters may be connect` The objects andadvantages ofthe invention will be better understood from the followingdef;

tailed description thereof when read in connection with the accompanyingdrawing in which,

Fig. 1 shows a push-pull electronic vacuum tube amplifier employingdirectly heated lament type cathodes operated on separate batteries andequipped with automatic plate current balance circuits in accordancewith the invention.

Fig. 2 shows the circuit of Fig. 1 modified to employ separate windingson an alternating current power transformer to heat the vacuum tubecathodes, and,

Fig. 3 shows the circuit of Fig. l modied to employ vacuum tubes havingindirectly heated cathodes operated either from a common battery or fromthe same winding of a power transformer.

The circuit of Fig. 1 comprises a source of alternating current I, aninput transformer 2 having a center tapped secondary winding forapplying energy symmetrically to the grids ofthe vacuum tube amplifiers3 and 4, and an output transformer 5 for coupling the plate circuits ofthe vacuum tubes'to a load resistance Ii. vPlate voltage is supplied tothe vacuum tubes 3 and 4 by a battery.. 'I through the centertappedprimary winding of the output transformer 5, and a fixedportion-ofthe grid bias is'furnished by a battery 8 through the centertapped secondary winding ofthe input transformer 2. Theautomatically'variable'portion of the grid bias on the vacuum tubes 3and 4 is supplied by the plate current flowing through the resistances9, III, II and I2. Condensers I3 and`l4whose impedances at signalfrequencies areV small compared with those of the `resistances 9, I0, IIand I2, serve to prevent degenerative action on waves passing throughthe amplifier. Separate batteries, "I5 and I6, are employed to heat thelaments of the vacuum tubes 3 and4.' l

'I'he operation of the circuitis as follows: It

s PATENroFFIc-E- f will first be assumed that thebias on the grids of ythe Vacuum tubes 3 and 4` due to the battery Y8 is such 'as to permit.appreciable plate current from thebattery 'I to ow through the'splitprimary Winding of the output transformer. 5, the plate to cathodeimpedances. of the tubes, and the resistances 9, IIL: II and I2. It willbe further assumed that .the grid .bias versus plate currentcharacteristics `of the vacuum tubes 3 and 4 are somewhat idiiierent sothat when identical bias voltages are applied to the grids, the platecur' rents flowing in the two tubesy arenot the same.

The plate current of the vacuum tube 3 from the-positive terminal of thebattery 1 divides at the junction of the resistances 9 and II, oneportionnowingfdownthrough the resistances IIY and I2 and up through theresistance I0, while thel other portion ilows down through theresistance 9. The two portions of the plate current of the vacuum tube 3reunite at the junction of the resistances 9 and I0 to ilow to thenegative terminal of the battery 1. The plate current of the vacuum tube4 from the positive terminal of the battery 'l divides at the junctionof the resistances I0 and I2, one portion `ilowing up through theresistances I2 and II and down through the resistance 9, While the otherportion ows up through the resistance I0. The two portions of the platecurrent of the vacuum tube 4 reunite at thejunction lof the resistancesI0 and 9 to flow to the negative terminal of the battery 1.

The portion of the bias due to the battery 8 is,

of course, the same on both grids of the vacuum tubes 3 and 4, so thaton the basis of the original assumptions, the plate current of tube 3,say, will initially be greater than that of tube 4. While this conditionholds, the plate current from tube 3 flowing down through theresistances Il and I2 will exceed the oppositely directed plate currentof tube 4 lflowing through thesame resistances. Thev cathode of tube 3thus becomes effectively more'positive with respect toA its grid whileat the same time the cathode of tube 4 becomes less positive. As aresult of these changes in bias, the plate current ofv tube 3 is reducedand that of tube 4 is increased so that they are more nearly equal thanthey would be if xed bias voltages of equal magnitude were applied toboth grids.

The circuit of Fig. 2 differs from that` of Fig. 1 only in that separatewindings I1 and I8 on a power transformer I9 are used to supply alter-vnating current to the directly heated Vfilament type cathodes of thevacuum tubes 3'and 4., The operation ofthe vcircuit of Fig. 2.isotherwise identical with that of Fig. 1. v The circuit of Fig. l3differs from that of Fig. 1 in that indirectly heated cathodes 2U and2ly are used instead of directly heated filament type cathodes. v Theheaters may have direct current supplied from a common batteryl 22y oralternatl, ing current from a common winding 23 of a power transformer24.,The operation of the circuit of Fig. 3' is likewise identical withthat of Fig. 1.

By a similar method of procedure, the plate current of the two tubes canbe shown to ap proach equality automatically when that of tube 4initially exceeds that of tube 3.

The .inventionis not limited to systems employing. three electrodetubes` but may be used with tubes having anydesired-.number ofelectrcdes. In multi-electrode tubes the space current may includecomponentsfrom `the several gridsas well as from the anode.

What is claimed is: j 1 1. In an electrical circuit, .vacuum tubesarranged in push-pull relation, each tube including.

a grid, a cathode, and .an anode.. means tol set up a voltage determinedin magnitude. and po-r larity by the difference between the spacecurrents of the tubes,. and means to impress por.. tions of said voltagein opposite polarity upon thev grids of the tubes to .substantiallyequalize the space currents.

2. In an electric circuit, vacuum .tubes arranged in` push-pullrelation, each tube vincluding agrid, a cathode, and an anode, means toinsure the same space current flow in each tube, comprising .electricalmeans for controlling the grid potential of each tube, said means beingactuated by a difference in the space current iiow in the tubes.

3. In an electric circuit, vacuum tubes arranged in push-pull relation,each tube including a grid, a cathode, and an anode, means to insureequal anode direct current flow in the tubes, comprising a separateresistance element in each anode' circuit, and means controlled by thedifference in the IR drops through the resistances due to the anodecurrents, said means controlling the grid potentials of the respectivetubes.

4. In combination with a pair of vacuum tubes in push-pull connection,impedances individual to the space current paths of the tubes and havinga terminal in common, a divided impedance bridged between the non-commonterminals of the individual impedances, a conductive connection from thegrid of each tube to the division point of the divided impedance, and aconductive connection from the cathode of each tube to the non-commonterminal of the impedance individual to such tube, whereby thepotentialof each grid with respect to its corresponding cathode is made to dependupon the diiference in the space currents in the respective tubes.

5. In combination with a pair of vacuum tubes in push-pull connection, apair of substantially equal resistances each individual to the spacecurrent path of one tube and having a terminal in common, a resistancebridged between the non-common terminals of the individual resistances,a conductive connection from the grid of each tube to the mid-point ofthe bridging resistance, and a conductive connection from each cathodeto they non-common terminal of the associated individual resistance,whereby the space currents of the respective tubes are automaticallymaintained substantially equal.

6. In coxnbnation'with a pair of vacuum tubes in push-pull relation, apairA of `substantially equal resistances having a terminal in commonand each resistance being individual to the space current path ofone'tube, a resistance bridged between the non-common terminals of theindividual resistances, a conductive connection from the grid of eachtube to the mid-point of the bridging resistance, and a conductiveconnection from each'cathode tothe non-common terminal of the associatedindividual resistance, whereby an' inequality of space currents in thetubes causes` a positive increment of potential to be impressed upon thegrid of the tube having the smaller space current and a negativeincrement upon the grid of the tube having the larger space current.

RAYMOND O. WISE.

CII

